Warm up and cool down are important.

How To Warm-Up To Improve Performance
A warm-up is the exercise of the body and mind before training or a match. It should be undertaken gradually. The key benefits from well-performed warm-ups are to:
-Reduce the chance of soft tissue injury by stretching muscles.
-Increase the body's temperature and blood flow.
-Allow mental preparation for the forthcoming exercise or game.
-Increase the level of performance.

Most coaches would agree that a warm-up session should also be enjoyable, varied and support the player as an athlete as well as a player. Here is a summary of the consensus view of sports scientists:
-Stretching for warm-ups should be dynamic not static. "Dynamic stretching" means stretches carried out through a controlled range of movements rather than held for a period of time.
-Warm-ups should be specific to the activity about to be undertaken.
-The cool down is as important as the warm-up and should also be performed after every training session or match.
-To derive the greatest benefits from a warm-up, the work should mirror the actions that are to be taken while competing.
-Specific warm-ups are best employed after completing a general warm-up, which reduces the chance of injuries.

Warm-up guidelines
-A light sweat over the entire body is the best indicator of the correct body temperature (around a one or two degree Celsius increase in temperature).
-Avoid letting your players get tired out.
-The benefits of the warm-up are lost after between five and 45 minutes of rest. Once a specific warm-up is completed the athlete should remain active .

The cool-down
After a match or training session it is essential to use some form of warm-down or cool-down. The benefits of a cool-down are that it helps to:
-Break down waste products in the muscles.
-Prevent muscle soreness by coaxing muscles gently back into shape.

A cool-down should include:
Gentle movements like jogging and skipping, and floppy "loose limb" movements.
Static stretching of the muscles for 10–15 seconds. Don't let your players overstretch. They are trying to help the repair process and if the body has been bashed around in a game, the muscles will need to be gently coaxed back into shape.

Summary:
Build the level of intensity gradually.
Ensure the technique is correct, even if the activity is light and gradual.
Increase the heart rate first then move to stretching and specific activity.
Have plenty of water available.
Take account of the weather conditions.

Credit goes to Dan Cottrell, editor of www.betterrugbycoaching.com. Visit the website for more tips on rugby.